1. Managing Email Routing
Module 9
In this module we will look at the techniques required to ensure that messages are delivered to their intended destinations.
After completing this modules, students will be able to:
Explain DNS’s role in mail services
Explain what an MX record is
Specify what MX record(s) a domain needs
Prioritize mail servers
Discover the mail servers for another domain
Configure a relay host
Managing Routing

2. Configuring MX Records
MX records in DNS instruct mail servers where to direct messages
example.com IN MX 10 elm.example.com. example.com IN MX 20 spruce.example.com. example.com IN MX 30 oak.example.com.
Properly configuring your domain’s MX records in DNS is essential to inform other mail servers where to send messages destined for your domain.
One of the fields in an MX record is the preference. In environments in which there is more than one mail server, the preference value is used to indicate the order in which the mail servers should be chosen. The highest priority is 0.
When a remote mail server sends a message to your domain, it is given all of your domain’s MX records. The remote mail server selects the highest priority host (i.e. your mail server with the lowest preference value). If the remote mail server cannot deliver to that mail server, the remote mail server tries the next highest priority host. If that fails, the remote mail server continues to try the next highest priority hosts base on your domain’s MX records until it is able to deliver the message.
domain name class type preference hostname

3. Specify “set querytype=MX”
Querying MX Records
When debugging problems exchanging with other domains, query MX records
Use nslookup(1)
Specify “set querytype=MX” 1 2 3 4
If problems occur when exchanging with other domains, the nslookup(1) command can be used to determine the name and IP address(es) of a domain’s mail server(s).
In the example nslookup(TC) output above,
By default, nslookup(TC) queries for A records. Here we indicate that we want to search for MX (mail exchanger) records by typing set querytype=MX.
We type the name of the domain whose mail servers we are querying for.
The command responds with the name and preference (a.k.a. priority) of each mail server in the example.net domain.
The command responds with the IP address of each mail server in the example.net domain.

4. Configuring a Relay Host
A relay host enables delivery to be centralized
In main.cf:
relay_host = oak.example.com or
relay_host =
In environments where administrators wish to centralize mail delivery, a relay host can be configured.
Configuring a relay host involves setting $relay_host in /opt/insight/etc/postfix/main.cf. $relay_host can be set to the name or the IP address of the gateway mail server.
After configuring $relay_host, run postfix reload.